Arrangements of oxygen atoms in YBa2Cu3O6+x superconductors visualized by ultra-high-resolution electron microscopy

Using an ultra-high-resolution high-voltage electron microscope (UHR-HVEM) recently constructed at NIRIM, the crystal structure of tetragonal YBa2Cu3O7.7, with a superconducting transition temperature of 92 K, was analyzed. Dark spots from oxygens were detected directly, in addition to those of Y, Ba and Cu atoms. As a result, it was confirmed that the structure is similar to that of the tetragonal YBa2Cu3O6+x (0<x<0.35) crystal, already known. Excess oxygens are accommodated almost homogeneously in otherwise vacant oxygen sites in CuOy planes. Furthermore, arrangements of oxygen atoms in a YBa2Cu3O6.4 crystal with Tc=27 K were examined. Oxygen atoms in CuOy planes do not distribute homogeneously but are mostly concentrated in areas smaller than 100 A in diameter, in which clusters of 8 to 30 A are closely packed with an oxygen arrangement similar to that in YBa2Cu3O7.7 or YBa2Cu3O6.9, respectively. The volume ratio of these two types of clusters varies in each oxygen-rich area, resulting in local change of the crystal symmetry. The formation of these areas as well as clusters can be related to the mechanism and properties of superconductivity. (orig.)